1. Field of the Invention
This invention relates to the processing of digital audio signals.
2. Description of the Prior Art
In many modern audio signal processing devices, such as audio mixing consoles, audio processing operations which had previously been carried out in the analogue domain are now performed on sampled digital audio signals.
An example of this is the "dynamics" section of a digital audio mixing console. Dynamics processing refers to a family of processing techniques generally having a non-linear effect on the audio signal (compared with the substantially linear techniques of simple gain adjustment and additive mixing). In general, effects classified as "dynamics" tend to have a distorting effect on the sound represented by the audio signal, albeit often a pleasing or useful distortion. For example, the gain applied to an audio signal might be non-linearly adjusted (or "compressed") so that the audio signal has a substantially constant level--alleviating the level variations which might result as a performer moves towards and away from a microphone.
FIG. 1 of the accompanying drawings is a schematic diagram of a previously proposed compression processor, with the compression response of this processor being illustrated schematically in FIG. 2.
In FIG. 1, an input digital audio signal (for example, a digital audio signal sampled at a 48 kHz sampling rate to 16 bit resolution) is supplied in parallel to a multiplier 10 and to a processing chain 20.
The processing chain 20 comprises a peak (or envelope) detector 30, a logarithmic (linear to decibel) converter 40, a dynamics processing element 50, a control circuit 60, and a logarithmic (decibel to linear) converter 70.
The output of the logarithmic converter 70 is supplied as a second multiplicand to the multiplier 10, to be multiplied by sample values of the input digital audio signal. In this way, the output of the processing chain 20 provides a gain control for the input digital audio signal.
The dynamics processing element 50 operates in the logarithmic domain, i.e. it receives envelope values and generates gain control values in decibels rather than as linear measures. This is so that the time constants, control values and other constants used by the dynamics processing element relate to a decibel law directly, which in turn makes the implementation of the dynamics processing element more simple and intuitive.
The dynamics processor of FIG. 1 may be arranged to provide various different dynamics processing functions, depending on the way in which the dynamics processing element 50 generates an output gain control value in response to the detected envelope of the input digital audio signal. For the present explanation, consider the simple example whereby the dynamics processing element is arranged as an "attack-hold-release" compressor. This type of compressor provides a relatively quickly increasing degree of compression when the input signal envelope suddenly increases in magnitude. Then, when the input signal is no longer increasing, the level of compression attained is held substantially constant during a "hold" period. Finally, during a "release" period, the compression is reduced towards zero compression. This example compression response is illustrated schematically in FIG. 2 of the accompanying drawings.
A perceived advantage of the use of digital techniques to implement compressors of this type is the opportunity to provide an accurately predetermined compression response--for example, holding the compression accurately at the same level throughout the "hold" period.
The gain control value output by the dynamics processing element 50 is then converted to a linear control value by the logarithmic converter 70 and supplied as a multiplicand to the multiplier 10.